One of the biggest challenges in semiconductor packaging is picking, handling and processing of very thin semiconductor chips. The semiconductor chips are generally provided on a carrier tape, which generally contains a whole semiconductor wafer that has been cut into small chips, a process frequently referred to as dicing. The semiconductor chips are thus commonly referred to as dies. The carrier tape is often the same tape that supported the wafer during dicing, often referred to as dicing tape. Die thicknesses of down to 25 μm are common today, and there is an ongoing trend to decrease thicknesses even further. Consequently, features on a structured side of such thin dies usually have maximum heights of only a few micrometers.
Die bonders pick a single die from the carrier tape and place and subsequently attach the picked die onto a substrate or onto another die. In most situations, the die is attached permanently, but configurations where dies are only attached temporarily also exist. Often, a temporary attachment is subsequently turned into a permanent one by an additional heating and/or pressing process. Often, thin dies come with a wafer backside lamination (WBL) or film or flow over wire (FOW) lamination, i.e. an adhesive film which is applied to an unstructured side of the wafer or die. A process of thinning wafers, applying an adhesive film to the wafers, mounting the wafers to a carrier tape and frame, and dicing them into individual chips is usually referred to as wafer preparation. The lamination, which allows for the dies to be attached due to its adhesive properties, may be provided either between the carrier tape and the wafer, or on a surface of the wafer facing away from the carrier tape.
Picking and placing may be carried out by a single die handling unit in the die bonder as illustrated in FIG. 1. In modern die bonders, e.g. as shown in FIG. 2, a plurality of die handling units can be present: In general, picking takes place from a so called wafer table, and is assisted by a first die handling unit, also referred to as die ejector 5. The die ejector 5 facilitates the removal of the die 30 from the carrier tape 4, e.g. by pushing the die 30 against a second die handling unit called pick unit 92—from underneath the carrier tape 4. The pick unit 92 subsequently picks the die 30 from the carrier tape 4 in a pick process and hands it over to a third die handling unit, also referred to as place unit 94. The place unit 94 places the die 30 onto a target position, where it is attached. In some cases, at least one fourth die handling unit—a so called transfer unit 93—is provided to hand the die 30 from the pick unit 92 to the place unit 94. An example is given in WO 07118511 A1 which is hereby incorporated by reference in its entirety. In this manner, a die 30 may be attached to a substrate 6, e.g. a leadframe, printed circuit board, multilayer board etc., or to another die, which itself may be have been attached in the same way. A plurality of cameras 40, 41, 42, 44 is normally present for monitoring and/or measuring purposes.
The fabrication of very thin wafers and the corresponding dies is very expensive as compared to standard thickness wafers without lamination. Sawing, picking as well as handling of very thin dies have significant yield losses. Most of the yield is lost during wafer preparation, die picking, or subsequent handling, resulting in damaged dies due to typical defects as e.g. broken dies, cracked dies, chipped dies, etc. Both place and attachment processes, in comparison, are more reliable, giving rise to only negligible loss.
During picking, damage frequently results from bending a die beyond an elastic range. This occurs when the die 30, while still being attached to the carrier tape 4 is grabbed by a pick tool 1 and subsequently detached from the carrier tape 4 as shown in FIG. 3. This is usually done by bringing a work surface of the pick tool into contact with a first surface of the die 30 which faces away from the carrier tape 4, fixing the die 30 to the work surface by means of a vacuum, and moving the pick tool 1 away from the carrier tape 4 which is thus peeled off of the die 30. During peeling, regions of the carrier tape 4 underneath positions of neighboring dies 31, 32 are generally held in place on a support surface of a die ejector 5 by vacuum. As a consequence, the carrier tape 4 exerts a reactionary force onto the die 30 away from the work surface. As air can easily enter between the die 30 and the work surface at an edge of the die 30, vacuum often breaks down in an area near the edge so that the die 30 looses contact to the work surface in said area. The area near the edge of the die 30 is then bent down. This may lead to breaking, cracking or chipping of the die 30 if a bending radius becomes too small. It may also result in the die 30 becoming completely detached from the pick tool 1. In this case, picking has to be repeated, resulting in reduced performance of the die bonder.